The invention is a continuous excavation/demolition system based upon the controlled fracturing of hard competent rock and concrete through the controlled application of a high-pressure foam-based fluid in pre-drilled holes.
For over a century explosive blasting has been the primary means used for the excavation of hard rock and often the demolition of concrete structures. In recent years several small-scale methods employing small explosive or propellant charges or specialized mechanical and hydraulic loading means have been proposed as alternatives to conventional blasting. Conventional blasting is limited in that it requires special precautions due to the use of explosives and that it can cause excessive damage to the rock or concrete being broken. The smaller scale specialized techniques, while finding many niche applications, have been limited in their ability to break harder rocks or in having undesirable operating characteristics. For example, the small-charge explosive and propellant techniques still generate significant airblast and fly rock.
Efforts to develop alternatives to conventional explosive excavation and demolition have included water jets, firing high velocity slugs of water into predrilled holes, rapidly pressurizing predrilled holes with water or propellant generated gases, mechanically loading predrilled holes with specialized splitters, various mechanical impact devices and a broad range of improvements on mechanical cutters. Each of these methods may be evaluated in terms of specific energy (the energy required to excavate or demolish a unit volume of material), their working environment, their complexity, their compatibility with other excavation operations, and the like.
The specific energy required to excavate rock or demolish rock or concrete with any existing technique is found to be extremely high as compared to the energy required to form the fractures needed to achieve the desired breakage. For example, rocks have a laboratory determined fracture energy ranging from 10 to 500 Joules per square meter, this being the work (energy) required to create the two faces of a new fracture. Taking 100 J/m.sup.2 as representative and requiring that the rock be broken into 1 mm (0.001 m) fragments dictates that 300,000 Joules per cubic meter of material be expended on fracturing alone. In contrast, conventional drill and blast requires an expenditure, including drilling of the shot holes, of 30,000,000 Joules per cubic meter (30 MJ/m.sup.3) and conventional drilling and tunnel boring machine operations require on the order of 300 MJ/m.sup.3.
The energy expended in all existing methods of excavation and demolition exceeds the energy needed to accomplish the desired result by 100 to 1000 orders of magnitude. This very large difference indicates that the existing methods are quite inefficient.
Controlled fracture methods, in various forms, have been proposed for several years as means to excavate or demolish rock and concrete more efficiently. Denisart (1976) proposed the rapid pressurization of a predrilled hole by firing a steel piston into a water filled hole such that a preferred (controlled) fracture would be initiated at the hole bottom and by propagating back to the surface from which the hole was drilled would efficiently remove a volume of the material.
Lavon (1978, 1979, 1980a and 1980b) proposed a variety of hydraulic cannons such that a high-velocity slug of liquid (water) could effect an efficient fracturing, excavation or demolition upon being fired into a predrilled hole.
Alternative methods for fracturing rock with hydraulic fluid pressure have been proposed by Cheney (1981) and Oudenhoven (1983). Cheney proposed placing a barrel type device with a mechanical (wedge and feather) collet to hold the device in the hole and a separate resilient sealing member (of elastomer, for example) into a pre-drilled hole and then pressurizing the bottom of the hole with a relatively incompressible fluid such as water so as to fracture the material to be broken. Oudenhoven proposed a very similar approach, but stipulated the cutting of a notch or groove near the bottom of the hole to assist in fracture initiation. Oudenhoven also proposed utilizing a single elastomer type of seal to hold the device in the hole and to provide for reasonable hole sealing. Neither Cheney nor Oudenhoven foresaw the possible use of foam as the fracturing fluid nor did they foresee the use of a seal of a deformable granular or cementitious material.
Cooper (1978) proposed a mechanical splitter such that both radial (perpendicular to the axis of a hole) forces and axial forces could be exerted upon a predrilled hole so that fracture would be initiated near the hole bottom and would propagate essentially parallel to the face from which the hole was drilled. Additional research and development on the radial-axial splitter has been carried out by the U.S. Bureau of Mines (Anderson and Swanson, 1982). The radial-axial splitter is limited in that the breaking forces are only applied to the sides and bottom of the drilled hole and are not applied to the fracture surfaces as the fractures develop. As fracturing must thus be accomplished without the benefit of fracture pressurization, the required stresses are much higher than needed for the fluid pressurization methods.
Realizing the benefits that might be achieved with the controlled fracturing of a material with a properly applied controlled pressure, Young (1990, 1992) proposed the use of small propellant charges to provide the requisite pressurization of a predrilled hole. Young noted that such pressurization would have to be restricted to the bottom of the hole by appropriate sealing means but that when such sealing was achieved a characteristic fracture would form at the sharp corner of the hole bottom. This characteristic fracture would initially propagate down into the material but would then turn back to the surface from which the hole was drilled as free surface effects began to control fracture propagation. The resulting breakage often left a cone on the rock face with the bottom of the predrilled hole defining the top of the cone. The method has since come to be known as the Penetrating Cone Fracture (PCF) method.
Propellants have been proposed earlier for the breaking of softer rocks such as coal (Davidson, 1956; Hercules, 1963 and Stadler et al, 1967) but these approaches did not envision the use of borehole sealing as used in the PCF method. Van Der Westhuisen (1990) also proposed a propellant based device for breaking boulders or other rocks with numerous free faces. As this device did not provide for any sealing near the hole bottom, it would not be expected to be efficient in excavating in-place rock.
Other propellant based rock fragmentation systems have been proposed by Watson and Young (1994), Ruzzi and Morrell (1995) and McCarthy (1997). Watson and Young provided for a high-strength cartridge which could be placed in a pre-drilled hole on the end of a stemming bar. The high-strength cartridge, by deforming to the borehole wall, would provide for the sealing and containment of the propellant gases near the hole bottom.
Ruzzi and Morrell provided for a mechanical (wedge and feathers) seal near the bottom of a pre-drilled hole such that the gases generated by the ignition of a propellant cartridge positioned on the end of the stemming/sealing bar would be contained near the hole bottom. McCarthy proposed a method for rapidly displacing a propellant cartridge to the bottom of a pre-drilled hole such that the propellant is ignited when the cartridge strikes the hole bottom. None of these three methods provide for the degree of hole bottom sealing required for effective breakage, especially if breakage is limited to one free face (the face into which the hole is drilled).
A high-pressure water injection device has been proposed by Kolle and Monserod (1991) and the rapid discharge of electrical energy from a high-voltage capacitor has been proposed by Nantel et al (1990). Again neither approach stipulated any sealing near the hole bottom. Breakage from the high-pressure water injection device is limited by the limited expandability of water as compared to a gas and the associated limits upon maintaining adequate fracture pressurization. Breakage from the electrical discharge device is limited by the rapid quenching of the electrical discharge generated gases once the gases (essentially steam) enter the rock fractures resulting in loss of adequate pressure for efficient fracturing.
The propellant techniques may have the advantage of providing a high-pressure gas for controlled pressurization but are hindered by the fact that the low viscosity of these gases require that the breakage process be completed in a very short period of time (before the gases can escape) which requires that the initial gas pressures be quite high, on the order of 300 MPa (45,000 psi) or higher. These high pressures result in significant airblast and fly rock which detract from the utility of the process. The propellant gas methods have the advantage over the water/steam pressurization methods in that the gases can expand as they flow into a developing fracture system and thus maintain their ability to adequately pressurize fractures. The propellant gases are comprised primarily of carbon monoxide, however, which requires special ventilation considerations in restricted or underground situations.
The excavation of hard rock for both mining and civil construction and the demolition of concrete structures are often accomplished with conventional explosives. Due to the very high pressures associated with explosive detonation these operations are hazardous, environmentally disruptive, require considerable security, protection of nearby personnel and equipment and must often be applied on an inefficient cyclic basis (as in conventional drill-blast-ventilate-muck operations).
Efforts to develop continuous and more benign excavation/demolition methods has been ongoing due to persistent problems in the industry. The PCF (Penetrating Cone Fracture) method using small propellant charges has proven the most promising to date. However, the PCF method is most limited as it still generates considerable airblast and fly rock, and as the propellant reaction gases may be comprised of over 50 percent carbon monoxide, a poisonous gas. The strength of the PCF method as compared to the other small-charge, electrical discharge and water cannon methods lies in that the propellant gases are able to maintain sufficient pressure for fracturing as the fracture system grows and increases in volume. It is the continuous and maintained pressurization of the developing fractures that enable the PCF method to work efficiently.
The present invention uniquely overcomes the limitations of all the above excavation/demolition methods. The present invention s hows that the proper pressurization of preferred or controlled fractures is the most efficient way to excavate or demolish rock and concrete.